What is the 4th coordinate of spline points?
NURBS curves are a set of weighted control points.

Control Point, Vertex
In case of a NURBS curve, there is a fourth component available (W),
which defines the weight of the selected control point or the median
weight.

For further technical details see https://en.wikipedia.org/wiki/Non-uniform_rational_B-spline#Technical_specifications

A NURBS curve is defined by its order, a set of weighted control
points, and a knot vector.[4] NURBS curves and surfaces are
generalizations of both B-splines and Bézier curves and surfaces, the
primary difference being the weighting of the control points,

How to specify Nurbs path vertices in python?
Create curve from Numpy Array using Python